NATURAL ENEMIES: BIRDS. 25 
The chimney swift, a distinctly insectivorous bird, shows 13 out 
of 139 to have eaten leafhoppers, but one of these showed 50 per cent 
jassid material. 
The flycatchers, which are preeminently insect feeders and active 
in the meadows and pastures, show, nevertheless, a very small leaf- 
hopper diet. The best record is for a California species (Empidonax 
difficilis) for which 11 birds in a total of 148 had eaten leafhoppers 
and in proportions as high as 73 per cent. Among our eastern 
species, only 3 birds in 91 of the yellow-bellied flycatcher (Empidonax 
Jlaviventris) had eaten leafhoppers, and in the proportion of 3 to 15 
per cent. 
Traill's flycatcher (Empidonax trailli) shows 5 in 134 with proportion 
for these up to 10 per cent, the Acadian flycatcher (E. virescens), 3 in 
93 with 10 per cent for two, and least flycatcher (E. minimus), 4 in 162 
with 10 per cent of jassid material in two and 20 per cent in one. 
The record for these four eastern flycatchers which might be expected 
to be especially serviceable shows, therefore, all together only 15 out 
of 480 stomachs to include jassid material, and the average for these 
can not be estimated as more than 10 per cent, so that the ratio of 
jassid diet would be only 1 to 500, or one-fifth of 1 per cent. The best 
that can be claimed for them, therefore, is that in case of excessive 
numbers of leafhoppers they might help a little in their destruction. 
Even less useful in this connection are the phoebe and pewee, 
showing only about 2 to 100 with jassid contents, though two stom- 
achs contained, one, 88 per cent, and another 100 per cent. The 
common kingbird, so universally present in fields, shows still less, 
6 in 634, one bird, however, showing 52 per cent. 
One of the best showings is made by a California species, Myiarchus 
cinerascens, for which 7 out of 90 birds had eaten jassids and in 
proportions as high as 94 per cent, while the eastern crested flycatcher 
(Myiarchus crinitus) shows again only 3 in 244 and a proportion of 
5, 15, and 50 per cent. 
Among the family of orioles and blackbirds, the cowbird shows the 
best record as a jassid feeder, there being 25 stomachs out of 590 
with jassid contents, and for these 25 the proportion varies from a 
trace to 61 per cent, the average percentage for the 25 stomachs being 
18.5 per cent — a food ratio of approximately 1 to 128; that is to say, 
1 bird out of every 23 had eaten jassids to the extent of nearly one- 
fifth of his bill of fare. It is fair to assume that many of the birds 
showing no jassid diet were taken at times or places where this food 
was not available, and on this basis we can fairly credit the cowbird 
with good service — the best apparently of any of the birds for which 
data have been examined except the sharp-tailed sparrow. 
The California redwing (Agelaius gubernator) shows a quite excel- 
lent record of 12 to 200, but our eastern species (Agelaius plianiceus) 
